1. Field of the Invention
The present invention relates to a loop-type heat pipe used as a heat conveying device for aerospace use, industrial use, and household use.
2. Description of the Related Art
FIG. 14 is an explanatory diagram illustrating the configuration of a conventional loop-type heat pipe disclosed in U.S. Pat. No. 4,765,396. FIG. 15 is a radial cross-sectional view illustrating the evaporator shown in FIG. 14. In the drawings, reference numeral 1 denotes an evaporator which is comprised of an evaporator container 3 having grooves and ridges 20 on its inner wall surface; a wick 2 provided in contact with the ridges 20; vapor channels 4 in the grooves between the wick 2 and adjacent ridges 20 of the evaporator 3; and a sump 5 surrounded by the wick 2 to accumulate a liquid-phase working fluid. Reference numeral 6 denotes a vapor pipe for introducing a gas-phase working fluid 10 into a condenser 7. Numeral 8 denotes a liquid pipe for recirculating the liquid-phase working fluid into the evaporator 1. Numeral 9 denotes an arrow indicating the flow of heat to be applied, 10 denotes an arrow indicating the flow of the gas-phase working fluid, 12 denotes an arrow indicating the flow of heat flowing out from the condenser 7, and 13 denotes an arrow indicating the flow of the working fluid which has been converted to the liquid phase after condensation of the gas-phase working fluid. As the wick 2, a polyethylene thermoplastic having uniform pores with a pore diameter of 10 to 12 mm in its entirety is used.
A description will be given of the operating principle of the conventional loop-type heat pipe constructed as described above. As shown by the arrow 9 indicating the flow of heat, the heat applied to the evaporator 1 is transmitted to the evaporator container 3 and causes the liquid-phase working fluid to evaporate in portions of contact 14 between the wick 2 and the ridges 20 of the evaporator container 3. The gas-phase working fluid 10 flows into the condenser 7 through the vapor channels 4 and the vapor pipe 6. As shown by the arrow 12 indicating the flow of heat flowing out from the condenser 7, the gas-phase working fluid which flowed into the condenser 7 is cooled, condenses, and is liquified and converted to a liquid-phase working fluid 15.
As shown by the arrow 13, the liquid-phase working fluid 15 passes through the liquid pipe 8 and is recirculates to the evaporator 1. The liquid-phase working fluid 15 which returned to the evaporator 1 is accumulated in the sump 5. The liquid-phase working fluid 15 is conveyed to the portions of contact 14 between the wick 2 and the ridges 20 of the evaporator container 3 by capillary action, and is vaporized and converted to a gas-phase working fluid by the heat absorbed by the evaporator 1.
In the above-described conventional loop-type heat pipe, if the volume of the sump 5 becomes large, the diameter of the sump 5 also becomes large. As shown by the arrow 16 in FIG. 14, the liquid-phase working fluid 15 accumulated in the bottom portion of the sump 5 permeates toward the upper portion of the wick 2 along the circumference of the wick 2, so that if the diameter of the sump 5 becomes large, the diameter of the wick 2 also becomes large. Hence, the liquid-phase working fluid has difficulty flowing toward an upper portion 17 of the evaporator container 3. There has been a problem in that if the rate of permeation of the liquid-phase working fluid into the wick 2 and the rate of evaporation from the wick 2 become off-balance, variations occur in the temperature distribution of the wick 2 and overheating proceeds locally, so that the heat conveying capacity which is obtained by a predetermined temperature difference between the evaporator 1 and the condenser 7 declines.
In addition, if the amount of heating of the evaporator 1 becomes large, the evaporation in the wick 2 proceeds before the liquid-phase working fluid 15 uniformly permeates the wick 2, and the working fluid, particularly in the upper portion of the wick 2, is completely evaporated. In that case, the gas-phase working fluid 10 in the vapor channels 4 flows backward from the wick 2 into the sump 5. As the result of the fact that the gas-phase working fluid which flowed backward through the wick 2 increases the pressure within the sump 5, the liquid-phase working fluid 15 condensed in the condenser 7 fails to be recirculated to the sump 5, thereby stopping the function of the overall loop-type heat pipe.
In addition, in the operation of this apparatus, the pressure within the vapor channels 4 becomes the highest, and the pressure within the sump 5 becomes the lowest. A capillary pressure difference .DELTA.Pc due to the wick 2, which serves as the driving force for liquid circulation, is produced between the vapor channels 4 and the sump 5, and a force due to this capillary pressure difference is constantly applied between the outer surface and inner surface of the wick 2. This capillary pressure difference .DELTA.Pc is expressed by the following formulae by using the pore diameter Rp of the wick and the surface tension s of the working fluid: EQU .DELTA.Pc=2.sigma./Rp (1)
As shown by this formula, the smaller the pore diameter Rp of the wick 2, the greater the capillary pressure difference .DELTA.Pc. That is, the smaller the pore diameter, the greater the force applied between the outer surface and inner surface of the wick 2, and this force causes the wick to be depressed toward its interior. As a result, there has been a problem in that contact at the portions of contact 14 between the outer surface of the wick 2 and the groove ridges 20 becomes incomplete, which hampers the smooth heat exchange of the liquid-phase working fluid 15 permeated in the wick 2.
Further, there has been a problem in that if the inside diameter of the sump 5 is made small to allow the liquid-phase working fluid 15 to uniformly permeate the overall wick, the internal volume of the sump 5 becomes small, making it impossible to accumulate the predetermined liquid-phase working fluid required.
In addition, unless portions of contact 18 between an end of the evaporator container 3 and the wick 2 are completely sealed, the gas-phase working fluid 10 which evaporated in the portions of contact 14 between the wick 2 and the ridges 20 of the evaporator container 3 flows backward from the portions of contact 18 into the sump 5, thereby increasing the pressure within the sump 5. As a result, there has been a problem in that the recirculation of the liquid-phase working fluid 15 condensed in the condenser 7 into the sump 5 is hampered, stopping the function of the loop-type heat pipe.
Further, there has been a problem in that if the liquid-phase working fluid 15 in the sump 5 evaporates by coming into contact with a contact surface 19 of the heated evaporator container 3, the pressure within the sump 5 rises, and the recirculation of the liquid-phase working fluid, which radiated heat and condensed in the condenser 7, into the sump 5 is hampered, thereby stopping the function of the loop-type heat pipe.
In addition, when heat enters only from one side of the evaporator container 3, if the evaporator container 3 has a hollow cylindrical shape, the evaporation of the liquid-phase working fluid in the wick is concentrated on one side, and the pressure loss in the wick becomes large, so that there has been the problem that the heat conveying capacity declines.
In addition, since the fabrication of the hollow cylindrical wick 2 is difficult, there has been a problem in that its fabrication cost is high.
In addition, the heat which was conducted from the evaporator container 3 to the sump 5 through the wick 2 heats and vaporizes the liquid-phase working fluid 15 in the sump 5, and produces the gas-phase working fluid in the sump 5. Since the temperature of the liquid-phase working fluid 15 which passed through the liquid pipe 8 and recirculated to the sump 5 is low, the gas-phase working fluid in the sump 5 undergoes heat exchange at its surface of contact with the liquid-phase working fluid 15 as well and is returned to the liquid-phase working fluid by undergoing a change in its phase. However, since the area of contact between the gas-phase working fluid in the sump 5 and the liquid-phase working fluid recirculated from the condenser 7 is limited to only the surface of the liquid-phase working fluid 15 in the sump 5, the efficiency in heat exchange is poor, and the pressure within the ll 5 rises. There has been a problem in that the rise in the pressure within the sump 5 hampers the circulation of the working fluid in its liquid phase or gas phase, possibly stopping the function of the loop-type heat pipe.
As shown in Formula (1), the smaller the pore diameter Rp of the wick 2, the greater capillary pressure difference .DELTA.Pc can be obtained. In the conventional example, since a wick having a large pore diameter Rp of 10 to 12 .mu.m is used, the capillary pressure difference .DELTA.Pc becomes small, with the result that there has been a problem in that the heat conveying capacity becomes small.
In addition, since the heat conductivity of the wick 2 used in the conventional example is small, the heat absorbed by the evaporator container 3 is not conducted efficiently to the wick 2. There has been a problem in that, in that case, the efficiency in heat exchange between the evaporator container 3 and the wick 2 permeated by the liquid-phase working fluid 15 is low.